From the pics on your other thread
it appears that the rivets cannot be extracted without ruining the retainer clip. If there is no access to the retainer clips under the seat, the only way to remove the rivets would be to either pry them out, or grind off or drill out the heads, none of which are simple or risk-free alternatives. In other words, the chairs were not designed to be disassembled.

I have been confronted by the same problem, same chairs..
I had the opportunity to 8 chairs, some had problems to the seat
others to the back.
I was contemplating to switch out parts and make 4 good ones, out
of 8 broken ones, but the rivets don't allow this.

I took some of them apart (drilled the rivets), sanded the steel parts and remounted it. The rivets of the seat are standard dimensions, you can find them.

I had them spray painted after putting them back together. If you want to do it right, and powdercoat the steel parts seperately, be carefull with the rivets, you don't want the paint to jump off. You could consider using aluminium rivets instead of steel. (The new revolt chairs are aluminium, I believe)

How are you measuring those rivets?
Rivets are sized by the diameter of the hole into which they fit, not by the diameter of their heads.

I'm not clear on what you're trying to do, exactly, but if it involves removing the rivets that are currently holding the chair together, you might want to leave the job to a professional (or to a skilled amateur who's done it before); removing and replacing rivets isn't a trivial procedure.

this is how I measured them
The hole is 7mm diameter, and the rivet lenght (without head) is 20mm. The diameter of the head is 14mm.
It seems like these are rather uncommon big rivets, as the max diameter (of the hole)I can find is 4.2mm